NANO EXPRESS

Copper Selenide Nanosnakes: Bovine Serum Albumin-AssistedRoom Temperature Controllable Synthesis and Characterization

Peng Huang • Yifei Kong • Zhiming Li •

Feng Gao • Daxiang Cui

Received: 4 March 2010 / Accepted: 19 March 2010 / Published online: 3 April 2010

� The Author(s) 2010. This article is published with open access at Springerlink.com

Abstract Herein we firstly reported a simple, environ-

ment-friendly, controllable synthetic method of CuSe

nanosnakes at room temperature using copper salts and

sodium selenosulfate as the reactants, and bovine serum

albumin (BSA) as foaming agent. As the amounts of sel-

enide ions (Se2-) released from Na2SeSO3 in the solution

increased, the cubic and snake-like CuSe nanostructures

were formed gradually, the cubic nanostructures were

captured by the CuSe nanosnakes, the CuSe nanosnakes

grew wider and longer as the reaction time increased.

Finally, the cubic CuSe nanostructures were completely

replaced by BSA–CuSe nanosnakes. The prepared BSA–

CuSe nanosnakes exhibited enhanced biocompatibility than

the CuSe nanocrystals, which highly suggest that as-pre-

pared BSA–CuSe nanosnakes have great potentials in

applications such as biomedical engineering.

Keywords Copper selenide � Nanosnakes �Bovine serum albumin � Synthesis � Characterization �Mechanism � Biocompatibility

Introduction

Copper selenides (CuSe) are well-known p-type semicon-

ductors having potential applications in solar cells, optical

filters, nanoswitches, thermoelectric and photoelectric

transformers, and superconductors [1]. A lot of efforts have

been devoted to the synthesis of copper selenides micro-

and nanocrystallites with various morphologies, such as

particles [2], tubes [3], cages [4], and flake-like structures

[5]. There have been a few reports on the synthesis of

copper selenide 1D nanomaterials. For example, Cu2-x Se

nanowires with lengths of several micrometers and diam-

eters of 30–50 nm have been prepared by employing

selenium-bridged copper cluster as precursor in a chemical

vapor deposition (CVD) process [6]. Also synthesized are

arrays of copper selenide nanowires of mixed compositions

of Cu3Se2/Cu2-x Se or Cu2-x Se/Cu in various proportions

with lengths of several micrometers and diameters of 13–

17 nm by using porous anodic aluminum oxide film as

template [7]. However, to our knowledge, few reports are

closely associated with the environmental-friendly con-

trollable synthesis of 1D snake-like morphological CuSe

nanomaterials based on biomolecule-assisted synthesis. For

example, Munoz-Rojas et al. [8] synthesized Ag@PPy

nanomaterials that had snake-like shape and showed the

properties of bending and folding under hydrothermal

conditions while retaining the crystallographic coherence

of the silver core, which were highly suggested that snake-

like 1D nanomaterials might have some unique properties

and potential application.

In recent years, biomimetic synthesis has become a

hotspot [9]. For example, Yang et al. [10] reported bio-

mimetic synthesis of Ag2S [10], HgS [11], and PbS [12],

etc. in the bovine serum albumin (BSA) solution. These

synthesized 1D nanomaterials have unique electrical,

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11671-010-9587-0) contains supplementarymaterial, which is available to authorized users.

P. Huang � Y. Kong � Z. Li � F. Gao � D. Cui (&)

Department of Bio-Nano Science and Engineering, National Key

Laboratory of Nano/Micro Fabrication Technology,

Key Laboratory for Thin Film and Microfabrication of Ministry

of Education, Institute of Micro-Nano Science and Technology,

Shanghai Jiao Tong University, 800 Dongchuan Road,

200240 Shanghai, China

e-mail: dxcui@sjtu.edu.cn

123

Nanoscale Res Lett (2010) 5:949–956

DOI 10.1007/s11671-010-9587-0

optoelectronic, biological, and mechanical properties with

fundamental significance and great potential in applications

such as electrochemical storage cells, solar cells, solid-state

electrochemical sensors, semiconductive optical devices,

catalyst, superionic materials, and biomedical engineering

[13–16] and have attracted tremendous attentions from

researchers in the field of materials, micro-electronics, and

nanotechnology in recent years. However, how to fully use

the advantage of bionanomaterials such as DNA, RNA and

proteins, and metal nanomaterials as assistant media to

fabricate 1D nanocomposites with controllable shapes and

unique properties is still a great challenge. Up to date, few

reports are associated with application of CuSe nanoma-

terials in biomedical engineering.

Herein, we selected one-dimensional copper selenide

nanocrystals (CuSe) as research target, chose BSA as

assistant reagent, developed a simple, nontoxic, room

temperature, environmentally friendly method to synthe-

size controllably 1D BSA-wrapped copper selenide snake-

like nanocomposites, and investigated these as-prepared

products’ properties by UV–vis spectroscopy, high-reso-

lution transmission electron microscopy, selected-area

electron diffraction, energy dispersive spectroscopy,

Raman spectroscopy, and MTT method. We found that as-

prepared CuSe nanosnakes own some unique properties

and enhanced biocompatibility, the possible formation

mechanism of CuSe nanosnakes is also explored. Our

primary results show that BSA–CuSe nanosnakes have

great potential applications in biomedical engineering.

Experiments

Materials

All the reagents, including Cu(NO3)2, Na2SO3, and Se

powder, were from Sinopharm Company, China. BSA with

average molecular weight of about 68 KD was from

Xiamen Sanland Chemicals Company Limited, China. All

other reagents were from Sigma Inc. Human fibroblast cell

line was obtained from the American Type Collection

Company. RPMI 1640 medium containing 10% fetal calf

serum was from Gibco Company. Agarose was from Sigma

(St. Louis, United States). 3-(4,5-Dimethyl-2-thiazolyl)-

2,5-diphenyl-2H-tetrazolium bromide (MTT) was obtained

from Dojin Laboratories (Kumamoto, Japan).

Synthesis of CuSe Nanosnakes

The Na2SeSO3 solution was prepared by refluxing sele-

nium powder (5 mmol) and Na2SO3 (5 mmol) in distilled

water (200 ml) under nitrogen atmosphere for 24 h. In a

typical synthesis process, 5 ml of 25 mM copper nitrate

aqueous solution and 10 ml of 3 mg/ml BSA aqueous

solution were mixed under vigorous stirring at room tem-

perature (25�C). The mixed solution of the BSA–Cu2?

emulsion was kept static under nitrogen protection for 2 h.

Then, 5 ml of 25 mM Na2SeSO3 solution was added. The

color of the mixed solution rapidly changed to black. The

mixed reaction solution was kept static under ambient con-

ditions for 96 h, and then was separated by centrifugation at

15,000 rpm. The collected black solid-state products were

washed with double distilled water and ethanol for three

times and dried in a vacuum at room temperature for 24 h.

During the process of nanosnakes growth, four replicas of

the same experiment were run in parallel. Each replica was

terminated at different times such as 24, 48, 72, and 96 h. To

investigate the influence of BSA on the formation of copper

selenide nanosnakes, a control experiment was carried out,

copper selenide was prepared in the aqueous solution

without BSA, and other conditions and procedures were the

same as in a typical experiment.

Characterization of Synthesized BSA–CuSe

Nanosnakes

These synthesized BSA–CuSe nanosnakes were charac-

terized by a UNICAM UV300 spectrophotometer (Thermo

Spectronic, USA), high-resolution transmission electron

microscopy(HR-TEM, Hitachi H-700H, Hitachi, Japan),

selected-area electron diffraction, energy dispersive spec-

troscopy, a PerkinElmer LS55 spectrofluorimeter, Laser

Raman spectroscopy, and Fourier transform infrared (FT-

IR) spectroscopy (an FTS135 infrared spectrometer from

BIO-RAD, United States).

Cell Culture and MTT Analysis

Human fibroblast cell line was cultured in RPMI 1640,

containing 1 9 105 mU/ml of penicillin and 0.1 mg/ml of

streptomycin supplemented with 10% (v/v) FCS, at 37�C in

a humidified 5% CO2 and 95% air atmosphere for 48 h.

These cells were collected and added into 24-well plates at

the concentration of 5,000 cells/well and continued to

culture for 24 h. Then, the 100 ll CuSe nanocrystals

(20 lg/ml) and 100 ll BSA–CuSe nanocrystals(20 lg/ml)

were added into the 24-well plates, not added into the

control wells, and continued to culture for 3 days. MTT

(5 mg/ml) was prepared in PBS, and 20 ll was added to

each well, and the cells were incubated for 4 h at 37�C,

then the medium was removed, 200 ll dimethyl sulfoxide

was added to each well, and optical density (OD) was read

at 515 nm. The cell viability was calculated by the fol-

lowing formula: cell viability (%) = OD (optical density)

of the treated cells/OD of the nontreated cells. The per-

centage of cell growth was calculated as a ratio of numbers

950 Nanoscale Res Lett (2010) 5:949–956

123

of CuSe or BSA–CuSe nanosnakes-treated cells and con-

trol cells with 0.5% DMSO vehicle [17–19].

Statistical Analysis

Each experiment was repeated three times in duplicate. The

results were presented as mean ± SD. Statistical signifi-

cance was accepted at a level of P \ 0.05.

Results and Discussion

Synthesis and Characterization of BSA–CuSe

Nanosnakes

As shown in Fig. 1a, we can clearly observe the BSA–

CuSe nanosnakes with different lengths. We also observed

that the cubic copper selenide nanostructures were firstly

formed in Fig. 1b. As the reaction time increased, the cubic

copper selenide nanostructures gradually disappeared and

became the nanosnakes. The resultant nanosnakes gradu-

ally grew longer and longer.

Regarding the synthesis of one-dimensional BSA–CuSe

nanostructures, sodium selenosulfate (Na2SeSO3) was used

as Se source, which has been widely used to prepare

nanocrystallite selenides such as CdSe [20] and PbSe [21].

Lakshmi et al. [22] and Nair et al. [23] have successfully

prepared copper selenide (Cu2-xSe and Cu3Se2) thin films

by using Na2SeSO3 as Se source. Na2SeSO3 is much more

active than Se powder, because it reacts easily with Cu2?

ions at room temperature, is also less toxic, and, therefore,

is safer to use than Na2Se or H2Se [24]. Equations (1) and

(2) describe the reaction processes:

Na2SO3 þ Se���!reflex

Na2SeSO3 ð1Þ

Na2SeSO3 þ Cu2þ þ H2O ! CuSe þ 2NaNO3 þ H2SO4

ð2Þ

In the course of synthesis of 1D BSA–CuSe nanostructures,

BSA was used as the soft-template to control the nucleation

and growth of the nanocrystals, and also the dispersion and

stabilization of the nanocrystals in solvents. As well

known, BSA possesses a zwitterionic character at the iso-

electric point (pI 4.7), displayed reversible conformational

isomerization as the pH value changing [25]. BSA can bind

with different sites of a variety of cationic and anionic

groups, which makes possible utilization of BSA-decorated

nanomaterials in a variety of supramolecular assemblies.

For example, any conformational BSA can form covalent

adduct with various metal ions [26], such as Cu2?, Ni2?,

Hg2?, Ag?, and AuCl4-.

Potential Mechanism of BSA–CuSe Nanosnake

Formation

In order to clarify the mechanism of synthesis of CuSe

nanosnakes, we characterized the CuSe nanostructures by

UV–vis spectroscopy. Figure 2a shows the UV–vis

absorption spectra of pure BSA, BSA–Cu2?, and BSA–

CuSe. The pure BSA has a special absorption peak at

280 nm. The spectrum of BSA–Cu2? complex did not

display shift and enhancement of absorption peak at

280 nm, because the BSA protein can provide multiple

binding sites for Cu2?. The spectrum of BSA–CuSe

nanocomposites clearly showed the absorption peak shift

from 280 nm to 228 nm after the Na2SeSO3 solution was

added into the BSA–Cu2? solution, indicating that the Se2-

released from Na2SeSO3 and reacted with Cu2? forming

CuSe nanostructures. Figure 2b clearly shows that the

absorbance of BSA–CuSe nanostructures was markedly

enhanced, because more and more BSA–CuSe nanosnakes

were formed.

The BSA–CuSe nanosnake was also characterized by

using high-resolution transmission electron microscopy

(HRTEM), selected-area electron diffraction (SAED), and

Fig. 1 TEM images of BSA–

CuSe nanosnakes (a) and the

cubic copper selenide

nanostructures (b)

Nanoscale Res Lett (2010) 5:949–956 951

123

energy dispersive spectroscopy (EDS). Figure 3a–c show

representative TEM images of the BSA–CuSe nanosnakes

at the different reaction time such as 24, 48, and 96 h,

respectively. We can clearly observe that the well-dis-

persed nanostructures displayed different sizes, represent-

ing the different growth stages. Within the 24-h reaction

time, BSA–CuSe nanostructures mainly exhibited cubic

structure with average size of 30 nm. After 24 h, the BSA–

CuSe nanosnakes formed gradually, their sizes were about

130 nm in length and 12 nm in width. After 48 h, the cubic

nanostructures had little change, short rods appeared, and

the nanosnakes grew wide and long (Fig. 3b). When the

reaction time reached to 96 h, the cubic nanostructures

almost completely disappeared, and the nanosnakes grew

homogeneously up to about 200 nm in length, and 14 nm

in width (Fig. 3c, f). When the reaction time was over 96 h,

the sizes of nanosnakes were almost unchanged. As shown

in Fig. 3d, the single nanosnake exhibits good crystalline

and clear lattice fringes. The lattice fringe spacing was

0.172 nm, consistent with the interplanar spacing of the

(113) plane of cubic berzelianite (Cu2-xSe) crystallites.

Figure 3e is the corresponding SAED pattern, revealing

that the nanosnakes are crystalline and can be indexed to

berzelianite Cu2-xSe.

In order to investigate the typical growth stage of

nanosnakes, we used HR-TEM to observe the samples at

48-h reaction time. Figure 4a depicts the typical mor-

phology of the cubic BSA–CuSe nanocomposites revealing

that the peanut-like assemblies and shorter nanorods were

generated. As shown in Fig. 4b, the adjacent nanostruc-

tures attached without sharing a same crystallographic

orientation. The experimental lattice fringe spacing,

0.146 nm, is consistent with the interplanar spacing in

monoclinic Cu2Se. The connected nanoparticles were

rotated to find the common crystallographic orientation

(indicated by the white arrow) [27]. After the rotations

were finished, they fused to form almost a perfect short

nanorod (Fig. 4c). The lattice fringe spacing is 0.173 nm,

which was in agreement with the interplanar spacing of the

(113) plane of cubic berzelianite (Cu2-xSe) crystallites.

To understand the growth mechanism of nanosnakes, the

representative TEM images of the devourment of cubic

nanoparticles were recorded in Fig. 5a–c, and the prepared

nanosnakes also characterized by scanning electron

microscope (SEM) (see Supplementary Fig. 2). When the

cubic nanoparticles were captured by the nanosnakes, the

square boundary gradually fuzzed with the reaction time

and disappeared finally (shown by the white arrow in

Fig. 5a). Figure 5b, c showed the capture transient and the

devourment stage, respectively. Figure 5d–f recorded three

different parts of an individual nanosnake, including the

neck (D), the body (E), and the tail (F), whose lattice fringe

spacing is respectively 0.266, 0.101, and 0.155 nm. The

different parts have different crystallographic orientation

and steadily existed in BSA solution. The EDS spectrum

shows the presence of elements Cu and Se in the prepared

nanosnakes (see Supplementary Fig. 3). The peaks of C

and O element are due to the BSA. The Supplementary

Table 1 documents the weight percentage and atomic

percentage of silver and selenium elements of the measured

area, which showed that the atomic ratio of Cu and Se does

not match the stoichiometric molar ratio (Cu/Se) of copper

selenide exactly. The main reason is that the amount of

selenium in the BSA solution is excessive (see Supple-

mentary Table 1). According to the above phenomena, it

could be presumed that the nanosnakes are growing at the

expense of the colloidal particles in the Ostwald ripening

process, and BSA act as a stabilizing agent to modify the

new generated nanosnakes surface.

To clarify the formation mechanism of BSA–CuSe

nanosnakes, we also obtained the FT-IR spectra and Raman

spectra of pure BSA, BSA–Cu2?, and BSA–CuSe powders.

The FT-IR spectra and the data of the main peaks are shown

in Fig. 6a and supplementary Table 2. The IR peaks of pure

BSA at 3,430, 3,062, 1,652, and 1,531 cm-1 are assigned to

the stretching vibration of –OH, amide A (mainly—NH

stretching vibration), amide I (mainly C=O stretching

vibrations), and amide II (the coupling of bending vibrate of

N–H and stretching vibrate of C–N) bands, respectively.

The difference between the IR spectrum of pure BSA and

Fig. 2 a UV–vis absorption

spectrum of BSA, BSA–Cu2?,

BSA–CuSe at 24 h, BSA–CuSe

at 48 h, BSA–CuSe at 72 h,

BSA–CuSe at 96 h; b The

change of absorbance at 190 nm

952 Nanoscale Res Lett (2010) 5:949–956

123

that of BSA–Cu2? is obvious. The characteristic peak of

–NH groups disappeared, suggesting that there might be

coordination interaction between Cu2? and –NH groups of

BSA, which may play an important role in the formation of

CuSe nanosnakes. In addition, the new peaks of BSA–Cu2?

at 1,021 and 824 cm-1 might be contributed to the inter-

action of Cu2? and BSA. The strong peak at 1,383 cm-1 in

the BSA–Cu2?, and BSA–CuSe spectra is attribute to the

absorption of NO3-1, which was introduced by the addition

of Cu(NO3)2. Comparing the IR spectra of BSA–CuSe

with those of pure BSA, the characteristic peak of –OH

groups shifts to a high wavenumber of about 5 cm-1, and

the characteristic peak of –NH groups disappears. The

results indicate that the conjugate bonds existed between

the CuSe nanosnakes and –OH groups and –NH groups of

BSA.

Fig. 3 TEM images of BSA–CuSe nanosnakes obtained after

different aging time in the typical experiment: a 24 h, b 48 h, and

c 96 h, respectively. d HRTEM image of an individual nanosnake.

e SAED pattern in an area including many nanosnakes. f The

histogram of nanosnakes at 96 h

Fig. 4 TEM images showing oriented attachment of cubic copper

selenide in BSA solution for 48 h. a Low-magnification TEM image

of sample. b HRTEM image of two primary crystallites forming

‘‘peanut’’ or ‘‘chain’’ via oriented attachment. c HRTEM image of a

single nanorod after being fused together

Nanoscale Res Lett (2010) 5:949–956 953

123

Raman spectroscopy is used to investigate the changes

in the electronic properties of nanomaterials through the

special electron–phonon coupling that occurs under strong

resonant conditions. Therefore, Raman spectra are very

powerful to detect of the new chemical bonds. As shown in

Fig. 6b, the difference between the Raman spectrum of

pure BSA and that of BSA–Cu2? is obvious. The bands

C–H of BSA at 2,926 cm-1 disappeared, suggesting that

there might be coordination interaction between Cu2? and

BSA. Comparing the Raman spectra of BSA–CuSe with

those of pure BSA and BSA–Cu2?, the characteristic peak

of Cu–Se bonds at 250 cm-1 was found, which is consis-

tent with the standard Raman spectra of cubic berzelianite

(Cu2-xSe) crystallites(RRUFF ID: R060260.2). The above

facts highly suggested that the Cu2-xSe nanosnakes were

successfully synthesized in the BSA solution.

To further study the formation mechanism of the

nanosnakes in the BSA aqueous solution, the conformation

changes in the secondary structures of BSA in the reaction

system were determined by CD spectroscopy, which is a

valuable spectroscopic technique for studying protein and

its complex. The CD spectra of pure BSA, BSA–Cu2?, and

BSA–CuSe solutions are shown in Fig. 7. From the figure,

it can be seen that the CD curve of BSA–Cu2? solution is

similar to that of the pure BSA solution, while the CD

spectrum of the BSA–CuSe solution is different from that

of pure BSA. According to the result, it can be seen that

copper ions only induced the smaller deformation of the

Fig. 5 TEM images showing oriented attachment of copper selenide

nanosnakes in BSA solution for 48 h. a Low-magnification TEM

image of sample. b, c TEM images of two different devour stages of

copper selenide nanosnakes. HRTEM images of different parts of an

individual nanosnake: d the neck, e the body, f the tail, respectively

Fig. 6 a The FT-IR spectra of

(a) pure BSA, (b) BSA–Cu2?,

and (c) BSA–CuSe in BSA

solution for 96 h. b Raman

spectra (632.8 nm excitation) of

pure BSA, BSA–Cu2?, BSA–

CuSe in BSA solution for 96 h

954 Nanoscale Res Lett (2010) 5:949–956

123

BSA molecules in the BSA–Cu2? solution, whereas there

were bigger changes in the BSA conformation in the BSA–

CuSe nanosnake solution, resulting from the strong con-

jugate bonds between BSA and surfaces of the colloidal

nanosnakes. With the growth of CuSe nanosnakes, more

and more a-Helix were stretched and transformed into

b-Sheets, which could be contribution to the impairment or

break of hydrogen bonds.

According to the data mentioned above, we suggest one

possible mechanism model of CuSe nanosnake formation

based on use of BSA as soft-template, shown in Scheme 1.

The basic principle is attributed to that whose structure

decides whose function. BSA has reversible conforma-

tional isomerization in different pH condition, when pH

value is lower than 4.7, BSA undergoes another expansion

with a loss of the intra-domain helices (10) of domain I

which is connected to helix (1) of domain II and that of

helix (10) of domain II connected to helix (1) of domain III

[27, 28] Then, BSA has three reversible forms: N forms, F

forms, and E forms, which could bind with CuSe nano-

particles, finally result in the formation of different shapes

of CuSe nanostructures, for example, CuSe nanoparticles

bound with N forms formed the sphere nanostructures,

CuSe nanoparticles bound with F forms formed the cubic

nanostrctures, and CuSe nanoparticles bound with E forms

formed the nanosnakes, final CuSe nanostrcutures strongly

depend on the structures of BSA proteins under the reac-

tion condition.

Biocompatibility of CuSe Nanocrystals and BSA–CuSe

Nanosnakes

As shown in Fig. 8, as the culture days increased, the cell

viability decreased accordingly, the cell viability in BSA–

CuSe group was markedly higher than that in CuSe

nanocrystals group, there existed statistical difference

between two groups (P \ 0.01), which shows that

BSA–CuSe nanosnakes own better biocompatibility than

the CuSe nanocrystals.

Conclusions

In conclusion, CuSe nanosnakes were successfully syn-

thesized at room temperature using BSA as soft-template.

Regarding the potential mechanism of the phenomena, we

suggested a possible model: at first, the cationic Cu2? ions

were covalently adducted to BSA, as the amounts of sel-

enide ions (Se2-) released from Na2SeSO3 in the solution

increased, the cubic and snake-like CuSe nanostructures

were formed gradually. Secondly, the cubic nanostructures

were captured by the CuSe nanosnakes, the CuSe nano-

snakes grew wider and longer as the reaction time

increased. Finally, the cubic CuSe nanostructures were

completely replaced by BSA–CuSe nanosnakes. The pre-

pared BSA–CuSe nanosnakes exhibited enhanced bio-

compatibility than the CuSe nanocrystals, which highly

suggest that as-prepared BSA–CuSe nanosnakes have great

potentials in applications such as biomedical engineering.

Scheme 1 Schematic mechanism of synthesis of CuSe nanosnakes

using BSA as soft-template

Fig. 8 Effects of 20 lg/ml BSA–CuSe nanosnakes and CuSe nano-

crystals on Human fibroblast cells

Fig. 7 The CD spectra of a pure BSA, b BSA–Cu2?, and c BSA–

CuSe in BSA solution

Nanoscale Res Lett (2010) 5:949–956 955

123

Acknowledgments This work was supported by the National Natural

Science Foundation of China (No.20803040 and No.20471599), Chi-

nese 973 Project (2010CB933901), 863 Key Project (2007AA022004),

New Century Excellent Talent of Ministry of Education of China

(NCET-08-0350), Special Infection Diseases Key Project of China

(2009ZX10004-311), Shanghai Science and Technology Fund

(10XD1406100). The authors thank the Instrumental Analysis Center

of Shanghai Jiao Tong University for the Materials Characterization.

Open Access This article is distributed under the terms of the

Creative Commons Attribution Noncommercial License which per-

mits any noncommercial use, distribution, and reproduction in any

medium, provided the original author(s) and source are credited.

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